Heat recovery system and power generation system

ABSTRACT

In an improved system for recovering heat from a combustion gas produced by burning wastes, the combustion gas or combustible gas produced by partial burning of the wastes subjected to dust filtration in a temperature range of 450-650° C. at a filtration velocity of 1-5 cm/sec under a pressure of from −5 kPa (gage) to 5 MPa before heat recovery is effected. The dust filtration is preferably performed using a filter medium which may or may not support a denitration catalyst. Heat recovery is preferably effected using a steam superheater. The dust-free gas may partly or wholly be reburnt with or without an auxiliary fuel to a sufficiently high temperature to permit heat recovery. The combustion furnace may be a gasifying furnace which, in turn, may be combined with a melting furnace. If desired, the reburning to a higher temperature may be performed under pressure and the obtained hot combustion gas is supplied to a gas turbine to generate electricity, followed by introduction of the exhaust gas from the gas turbine into a steam superheater for further heat recovery. The system can raise the temperature of superheated steam to a sufficient level to enhance the efficiency of power generation without possibility of corrosion of heat transfer pipes by the combustion gas or combustible gas.

This is a division of U.S. patent application Ser. No. 08/726,403, filedOct. 3, 1996.

BACKGROUND OF THE INVENTION

This invention relates to a system for recovering heat from combustiongases or combustible gases produced by partial burning of combustibles.In particular, the invention relates to a heat recovery system that canbe applied to the treatment of municipal solid wastes (so-calledmunicipal wastes” or MW) or waste plastics.

The reduction of dioxins and the rendering of soot and dust innocuousare two essential requirements that must be met by recent wasteincineration systems. In addition, it has been proposed that new thermalrecycling systems be established that can treat wastes not only asmaterials to be disposed of but also as alternative energy sources.

Advanced power generation systems using municipal wastes have beendeveloped with a view to generating electricity at a higher rate ofefficiency than conventional systems in the process of burning solidwastes. According to a modified version of this system that utilizesreburning and superheating, the steam produced in a waste heat boiler issuperheated to a higher temperature with a clean hot combustion gasproduced by reburning combustion gas from a combustion furnace usinghigh-grade fuel of different origin, for example, kerosine or naturalgas. Such an independent superheater is used for the purpose ofenhancing the efficiency of power generation with steam turbines. Theadvanced system of power generation from municipal waste utilizing suchsuperheating method is under active development as being suitable forincineration facilities of a comparatively small scale.

Gases produced in the combustion of municipal wastes generally containHCl which is generated by the combustion of polyvinyl chloride, and ifthe surface temperature of heat transfer pipes for heat recovery exceedsabout 400° C., corrosion of these pipes due to HCl becomes pronounced.To avoid this problem, the temperature of superheated steam must be heldlower than 400° C., but as a result increased efficiency of powergeneration with steam turbines cannot be achieved.

However, a recent study has revealed that the main cause of corrosion ofheat transfer pipes is in fact the deposit of molten salts on the pipes.Municipal wastes have high concentrations of salts such as NaCl (m.p.800° C.) and KCl (m.p. 776° C.) and, as the combustion proceeds, thesesalts form a fume and are deposited on the heat transfer pipes, thetemperature of which is low. Since this deposit accelerates thecorrosion of the heat transfer pipes, the maximum temperature of thesuperheated steam that can be used in the existing power generationsystems using municipal wastes has been about 300° C., which will ensurethat the surface temperature of heat transfer pipes can be held belowabout 320° C.

Table 1 compares the features of various thermal recycling systems.Obviously, for successful high-efficiency power generation and RDF(refuse-derived fuel) power generation, the use of higher-gradematerials as heat transfer pipes is not sufficient and conditionspreventing the above discussed corrosion problem must first be realized.

TABLE 1-1 Power Generating Method Details Features Comments Conventionalpower The heat of combustion is recovered Steam pressure is low becauseOnce a superheated steam generation by a waste heat boiler to generatethe superheated steam temperature of 400° C. is electricity using backpressure steam temperature has conventionally assured, high steamturbines. been set to a low level. As a pressures also will be result,the power generating attained. efficiency is also low. In recent years,heated steam at a temperature of 400° C. has been attempted. Highlyefficient generation New material development have No additional load onthe The development of by mew material led to materials for incinerationenvironment, assist fuel is not materials resistant to moltendevelopment furnaces and superheaters that are required. salt corrosionencounters resistant to corrosive components both technical and economicsuch as hydrochloric acid which are difficulties. It is thereforegenerated in the combustion of more important to create refuse/wastes.This has led to conditions that will avoid improvements in steamconditions corrosion. and enhancement of power generating efficiency.

TABLE 1-2 Power Generating Method Details Features Comments RDF PowerGeneration The addition of lime and the like to As it is difficult togenerate Though hydrochloric acid the waste material to produce aelectricity at a high efficiency formation is decreased, the solid fuelnot only has the advantage in a small-scale plant, only measures againstmolten salt of helping to prevent putrefaction solid refuse material iscorrosion are practically at but also helps to create more produced. TheRDF is the same level as before. It favorable steam conditions with atherefore collected for is therefore necessary to view to achieve ahigher level of generating electricity at high create conditions thatwill power generating efficiency by efficiency in a large-scale plant.obviate corrosion as dechlorination and desulfurization. describedabove. Advance Refuse Power Combined cycle power generation The mosteffective practical use The use of large amounts of Generation with gasturbine. Power is is to introduc such a system in high quality fuels andthe generated with a gas turbine, and large-scale incineration systems.ecoonomic feasibility of the waste heat from the gas turbine is Thisprocess requires gas process are problems. The utilized to superheat thesteam from turbine fuel such as natural gas. key is whether the unitprice the refuse waste heat boiler. By this of produced electricity ismeans the efficiency of power increased. generation is enhanced.

TABLE 1-3 Power Generating Method Details Features Comments Reburning byuse of an This is included in an Advanced This method offers a high fuelThe use of large amounts of Additional Fuel Refuse Power Generatingsystem. utilization efficiency and is high quality fuels is The steamfrom the waste heat effective in small-scale expensive. The key is toboiler is superheated by using incineration plants. ensure that theprice at additional separate fuel in order to which the power is sold isenhance the power generating greater than the fuel costs. efficiency ofthe steam turbine.

The advanced systems of power generation from MW involve hugeconstruction and fuel costs and hence require thorough preliminaryevaluation of process economy. Deregulation of electric utilities is apressing need in Japan but, on the other hand, the selling price ofsurplus electricity is regulated to be low (particularly at night).Under these circumstances, a dilemma exists in that high-efficiencypower generation could increase fuel consumption and the deficit in aresultant corporate balance sheet. Some improvement is necessary from apractical viewpoint. Therefore, what is needed is the creation of aneconomical and rational power generation system that involves the leastincrease in construction cost and which also consumes less fuel, namely,a new power generation system that can avoid the corrosion problem.

The mechanism of corrosion is complicated and various factors areinvolved in the reaction. However, it can at least be said that the keyfactor in corrosion is not the HCl concentration in the gas, but whetheror not NaCl (m.p. 800° C.) and KCl (m.p. 776° C.) are in such anenvironment that they take the form of a fume (molten mist). These saltsare fused to deposit on heat transfer pipes and thereby acceleratecorrosion. The molten salts will eventually become complex salts whichsolidify at temperatures as low as 550-650° C. and their solidificationtemperatures vary with the composition (or location) of municipal wasteswhich, in turn, would be influenced by the quantity and composition ofthe salts.

These are major causes of the difficulties involved in the commercialimplementation of advanced or high-efficiency power generation systemsusing MW.

Table 2 lists representative causes of corrosion and measures foravoiding corrosion.

TABLE 2 Causes of Corrosion Corrosion-Preventing Method 1. Accelerationof corrosion due Use of medium-temperature ex- to high-temperatureexhaust haust gas region. gases 2. Chlorine-induced corrosion Creatingan environment with FeO + 2HCl → FeCllhd 2 + H₂O low levels of HCl, Cl₂and in- Fe₃ → 3Fe + C stalling the superheating pipes in Fe + Cl₂ →FeCl₂ such low-chlorine zones 3. CO-induced corrosion Ceating anenvironment with CO reacts with protective low CO levels (that is,creating layers on the heat transfer an oxidizing atmosphere) and in-surfaces with reduction of stalling the steam superheater. ferric oxide(making up such in these low-CO zones. layers). 4. Alkali-containingaccretion 1. Do not permit adhesion of depositing on the pipe wallsdeposits by wiping the pipe Acceleration of corrosion due surface with aflow of to deposits of alkali metal fluidizing medium (main- salts suchas sodium and tain a weakly fluidized potassium salts. bed). 2. Utilizethe heat of the fluidizing medium which has a temperature at which thealkali salts do not melt. 3. Remove dust particles in the exhaust gashaving a temperature at which the alkali salts are solidified and removethe chlorine salts (chlorides) and then use the cleaned exhaust gas.

The utilization of a medium-temperature region of exhaust gases perTable 2, is known to a certain degree. However, a superheated steamtemperature of only 400° C. can be recovered from an exhaust gastemperature of about 600° C. at which the salts will solidify. Hence,the method based on heat recovery from exhaust gases would not becommercially applicable to high-efficiency thermal recycling systemsunless the problems of corrosion of molten salts is effectively solved.

The methods of avoidance of corrosion which are listed in Table 2 underitems 2), 3) and 4-1) and 4-2) are considered to be effective if theyare implemented by using an internally circulating fluidized-bed boilersystem in which a combustion chamber is separated from a heat recoverychamber by a partition wall.

The internally circulating fluidized-bed boiler system is attractivesince “the fluidized beds can be controlled below temperatures at whichalkali salts will melt”. However, this method is incapable of avoidingthe resynthesis of dioxins.

As is well known, dioxins are resynthesized in heat recovery sections.Studies on methods of treating shredder dust and its effective use haveestablished a relationship between residual oxygen concentration and HClconcentration in exhaust gases in fluidized-bed combustion at 800° C.According to reported data, HCl concentration was about 8,000 ppm(almost equivalent to the theoretical) when the residual oxygenconcentration was zero, but with increasing residual oxygenconcentration HCl concentration decreased sharply until it was less than1,000 ppm at 11% O₂ (at typical conditions of combustion).

“Shredder dust” is a general term for rejects of air classification thatis performed to recover valuables from shredded scrap automobiles andthe like; shredder dust is thus a mixture of plastics, rubber, glass,textile scrap, etc.

The present inventors conducted a combustion test on shredder dust usinga test apparatus of 30 t/d (tons/day) and found that the concentrationof HCl was comparable to 1,000 ppm (i.e., similar to the above mentionedstudy). To investigate the materials balance of the chlorine content,the inventors also analyzed the ash in the bag filter and found that itcontained as much as 10.6% chlorine, with Cu taking the form of CuCl₂.

With regard to CuCl₂, it has been reported that this compound is relatedto the generation of PCDD/PCDF in the incineration processes and servesas a catalyst for dioxin resynthesis which is several hundred times aspotent as other metal chlorides (ISWA 1988 Proceedings of the 5th Int.Solid Wastes Conference, Andersen, L., Möller, J (eds.), Vol. 1, p. 331,Academic Press, London, 1988). Two of the data in such report are citedhere and reproduced in FIG. 5, which shows the effect of Cuconcentration on the generation of PCDD (o) and PCDF (Δ), and in FIG. 6,which shows the generation of PCDD (o) and PCDF (Δ) in fly ash as afunction of carbon content. The report shows that CuCl₂ and unburntcarbon are significant influences on the resynthesis of dioxins.

It should be noted that carbon tends to remain unburnt in theincineration process since combustion temperatures cannot be higher than1,000° C.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstancesand has as an object the provision of a heat recovery system and a powergeneration system that can enhance the efficiency of power generation bysufficiently increasing the temperature of superheated steam withoutinducing corrosion of heat transfer pipes by combustion gases and whichyet is capable of suppressing resynthesis of dioxins in a latter stage.

This object of the invention can be attained by a system for recoveringheat from combustion gases produced by complete burning of combustiblegases produced by partial burning of wastes, in which either of thegases is subjected to dust removal in a temperature range of 450-650° C.at a filtration velocity of 1-5 cm/sec under a pressure of from −5 kPa(gage) to 5 MPa before heat recovery is effected.

In the heat recovery system, dust removal is preferably performed usinga filter medium such as a ceramic filter which may or may not support adenitration catalyst. Heat recovery in the system may be performed usinga steam superheater. Thus, in the present invention, not only moltenalkali salts which will cause corrosion but also CaCl₂ (produced by thereaction CaO+2HCl→Cacl₂+H₂O) are removed as solidified salts by dustremoval in the temperature range of 450-650° C., and this contributes toavoiding corrosion of heat transfer pipes in the superheater by moltensalts and HCl. Further, the filter medium which may or may not support adenitration catalyst can also remove CuO and/or CuCl₂ which arecatalysts for dioxin resynthesis and, hence, the heat recovery system ofthe invention is also capable of suppressing the resynthesis of dioxinsin a latter stage.

In the invention, the combustion gas or combustible gas may be partly orwholly reburnt with or without an auxiliary fuel to a sufficiently hightemperature to permit heat recovery. The reburning of the combustiblegas may be performed by supplying air or oxygen-enriched air or pureoxygen to the gas. The reburning of the combustion gas with an auxiliaryfuel may be performed using the residual oxygen in the combustion gas.The combustible gas may be obtained by partial burning of wastes. Thecombustible gas may also be obtained by carrying out a gasificationreaction in a low temperature fluidized-bed gasification furnace havinga fluidized bed temperature of 450-650° C. Thus, according to thepresent invention, the absence of molten salts contributes to avoidingthe corrosion of heat transfer pipes in the superheater which wouldotherwise occur at an elevated temperature if molten salts were presentand, as a result, steam can be superheated to a sufficiently hightemperature.

It should be noted that the gasification reaction which proceeds in areducing atmosphere reaction is not likely to generate CuO. In addition,unburnt carbon will hardly remain if complete combustion is performed at1,300° C. and above in a melting furnace subsequent to gasification.Therefore, the gasification and melting or slagging combustion system ofthe invention is the most rational method for suppressing theresynthesis of dioxins.

The object of the invention can also be attained by a heat recoverysystem and power generation system which is an extension of theabove-described gasification and slagging combustion system in thatcombustion or gasification, dust removal and reburning are performedunder pressure and that the combustion gas or combustible having anelevated temperature is supplied to a gas turbine for power generation,followed by the introduction of the exhaust gas from the gas turbineinto a steam superheater for heat recovery.

In the heat recovery method of the invention, the temperature of thecombustion gas or combustible gas can be lowered to 450-650° C. bycollecting heat in a boiler in a conventional manner, with the steamtemperature being below 300° C. and the surface temperature of heattransfer pipes being below 320° C. The temperature of superheated steamcan be raised to about 400° C. when the gas temperature is below 600° C.If desired, dust removal may be preceded by blowing powder of limestone,calcium oxide, slaked lime or the like into the combustion gas orcombustible so that they are reacted with the HCl in the gas. Thus, HClcan be removed sufficiently to ensure that the source of corrosion inthe combustion gas is further reduced drastically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet for a heat recovery system involving reburning inone embodiment of the invention;

FIG. 2 is a flow sheet for a heat recovery system employing thecombination of gasification and slagging combustion with reburning inanother embodiment of the invention;

FIG. 3 is a flow sheet for a heat recovery system employing thecombination of fluidized-bed gasification and slagging combustion withreburning by the firing of a combustible gas in accordance with yetanother embodiment of the invention;

FIG. 4 is a flow sheet for a combined cycle power generation plantemploying a two-stage gasifying system in accordance with a furtherembodiment of the invention;

FIG. 5 is a graph showing the influence of CuCl₂ concentration on thegeneration of PCDD and PCDF concentrations;

FIG. 6 is a graph showing PCDD and PCDF in fly ash as a function ofunburnt carbon content; and

FIG. 7 is a flow sheet for a test plant, with test data included, thatwas operated to evaluate the effectiveness of a medium-temperaturefilter for preventing the corrosion of heat transfer pipes whilesuppressing the resynthesis of dioxins.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Molten salts including, NaCl (m.p. 800° C.) and KCl (m.p. 776° C.) existas complex salts and exhibit a strong corrosive action when they aredeposited on heat transfer pipes. However, such complex salts aresolidified at 550-650° C., so most of such complex salts can be trappedif dust removal is performed at temperatures below the melting points(solidification points) thereof. Therefore, if the combustion gasresulting from the burning of municipal wastes is subjected to dustremoval at a temperature lower than the melting points of the complexsalts in the combustion gas, the heat transfer pipes installed at alatter stage can be prevented from being corroded by molten salts.

If the temperature of superheated steam is to be increased to 400-500°C. with a view to improving the efficiency of power generation, thetemperature of the combustion gas is desirably increased to at least600° C. or above. Dust-free combustion gas can be used as a high qualityheat source since it contains no salts. When the gas is reburnt with anauxiliary fuel such as natural gas by making use of the residual oxygenin the combustion gas, the consumption of the auxiliary fuel isremarkably reduced, as is the amount of resultant exhaust gas, comparedwith the heretofore proposed method of using an independent reburner andsuperheater in a power generating system.

The consumption of the auxiliary fuel can be reduced and an increase inthe amount of the exhaust gas can be suppressed by limiting the amountof the gas to be reburnt to the necessary minimum amount forsuperheating steam.

If the content of residual oxygen in the combustion gas is small,combustion air must be added. If oxygen enriched air or pure oxygen isused in place of combustion air, the consumption of the auxiliary fuelcan be suppressed and yet the temperature of the combustion gas can besufficiently increased while preventing an increase in the amount of theexhaust gas that need be treated.

If the waste is gasified under a deficiency of oxygen to producecombustible gas, such gas easily may be reburnt merely by supplyingoxygen-containing gas such as air at a later stage instead of using highquality auxiliary fuel, thus partly or completely eliminating the needto use the auxiliary fuel.

If the wastes contain copper (Cu), gasification thereof offers a furtherbenefit because in the reducing atmosphere, copper (Cu) is not likely toform copper oxide (CuO) which is known to function as a catalyst foraccelerating dioxin resynthesis. Hence, the potential of dioxinresynthesis in a later stage, is reduced If a fluidized-bed furnace isused in the gasification stage, the occurrence of hot spots can beprevented and operation in the low-temperature range of 450-650° C. canbe realized to accomplish a highly effective prevention of copperoxidation.

It should be noted that if the combustible gas has only a low heatvalue, oxygen enriched air or pure oxygen rather than combustion air maybe employed to decrease the consumption of the auxiliary fuel and yetincrease the temperature of the gas while suppressing the increase inthe amount of the combustion gas to be treated.

It should also be noted that the product gas from the gasifying furnacecontains a large amount unburnt solids and tar. If such a gas isdirectly passed through a filter, clogging may occur due to the unburntsolids and tar. To avoid this problem, part or all of the gas may beburnt in a high temperature furnace provided downstream of thegasification furnace before the gas is passed through the filter, sothat the temperature of the gas is elevated to a level that causesdecomposition of the unburnt solids and tar in the combustion gas. Thisis effective in solving filtering problems associated with the unburntsolids and tar. In addition, the combustion gas is heated to asufficiently high temperature to enable the decomposition of dioxins andother organochlorines in the combustion gas.

If the temperature elevation is performed in a melting furnace such thatthe produced gas is heated to a temperature level that causes melting ofthe ash content, the ash can be recovered as molten slag, and at thesame time the load on the filter can be reduced.

Another advantage of using a melting furnace is that any copper oxide(CuO) that may be generated in the gasification furnace can be convertedto molten slag, thereby further reducing the potential of resynthesis ofdioxins in a latter stage.

Ceramic filters are suitable for use as dust filters in the temperaturerange of 450-650° C. at a filtration velocity of 1-5 cm/sec under apressure of from −5 kPa (gage) to 5 MPa. For use at higher temperatures,ceramic filters of tube, candle and honeycomb types are currently underdevelopment, but those for use in the temperature range of 450-650° C.which is used in the invention are already in the stage of practicaluse. The honeycomb-type filter has the particular advantage that itprovides a sufficiently large filtration area per unit volume to enablethe fabrication of the filter unit in a small size. A problem with thistype of filter is that if the diameter of honeycomb cells is small, thechance of the bridge formation will increase, causing the need toperform frequent backwashing. If such a problem is anticipated, a systemcapable of reducing the load on the filter will be necessary and thecombination of the aforementioned gasifying and melting furnaces will beeffective. Needless to say, this system is also effective in the case ofmunicipal wastes having a high ash content.

If the honeycomb-type filter is used to remove the aforementioned copperchloride (CuCl₂) and copper oxide (CuO) to a fine dust level, thepotential of dioxin resynthesis at the latter stage can be reduced to aninfinitesimally small level.

The ceramic filters for use in the invention may be made ofalumina-based compounds such as mullite and cordierite, or highlycorrosion-resistant titanium dioxide. For operations in a reducingatmosphere, filters made of highly corrosion-resistant non-oxide baseceramics such as silicon carbide and silicon nitride may be used. Ifcatalysts such as vanadium pentoxide and platinum are supported on thesurfaces of the ceramic filters, not only the dust component in thecombustion gas but also nitrogen oxides and dioxins can be reduced.

The thus treated dust-free combustion gas or combustible not only is oflow corrosive nature, but also the potential of “ash cut”, or wear bydust, is sufficiently reduced to achieve a significant increase in thegas flow rate of the combustion gas or combustible gas inside a heatexchanger. As a result, the pitch of heat transfer pipes can be reducedand yet the heat transfer coefficient that can be achieved is improved,whereby the size of the heat exchanger is sufficiently reduced torealize a substantial decrease in the initial investment.

If combustion or gasification of the waste is performed under pressureand dust removal in the temperature range of 450-650° C., followed byintroduction of the hot combustion gas or combustible gas into a gasturbine, a combined cycle power generation is realized, leading tohigh-efficiency power recovery.

The present invention will now be described in greater detail withreference to the accompanying drawings.

FIG. 1 is a flow sheet for a heat recovery system involving reburning inone embodiment of the invention. A combustion furnace 1 is supplied withmunicipal wastes 10, which are combusted to generate a combustionexhaust gas. The gas is then supplied to a waste heat boiler 2, where itis cooled to 450-650° C. by heat exchange with heated water coming froman economizer 6. Recovered from the waste heat boiler 2 is saturatedsteam 20 having a temperature of about 300° C. and a pressure of about80 kgf/cm². Subsequently, the combustion exhaust gas is filtered in atemperature range of 450-650° C. at a filtration velocity of 1-5 cm/secunder a pressure of from −5 kPa (gage) to +2 kPa (gage) by means of amedium-temperature filter 3. In addition to the feed waste, thecombustion furnace 1 may be charged with a neutralizing agent 13 such aslimestone for absorbing HCl in the combustion exhaust gas. If necessary,a neutralizing agent 13 such as slaked lime may be introduced into aflue 12 connecting to the filter so as to remove directly from HCl theexhaust gas. Stream 14 which is part or all of the combustion exhaustgas exiting the medium-temperature filter 3 is supplied to a heatingfurnace, 4 where it is reburnt to a higher temperature with an auxiliaryfuel 15. The thus heated exhaust gas 16 is sent to a steam superheater5, where saturated steam 20 coming from the waste heat boiler 2 issuperheated to about 500° C. The combustion exhaust gas 17 goes to theeconomizer 6 and an air preheater 7 for heat recovery. Thereafter, theexhaust gas passes through an induced blower 8 and is discharged from astack 9. The steam 21 superheated in the steam superheater 5 is sent toa steam turbine 22 for generating electricity 28.

If the saturated steam 20 is directed into the waste heat boiler 2 wherethe exhaust gas temperature is below about 600° C. such that such steamis heated to a temperature about 400° C., saving of the auxiliary fuel15 can be accomplished.

Denoted by 11 and 18 in FIG. 1 are noncombustibles and water.

FIG. 2 is a flow sheet for a heat recovery system employing reburningcombined with gasification and slagging or combustion to insure completecombustion. As shown, municipal wastes 10 are gasified in a gasifier orgasification furnace 23 to generate a combustible gas, which is oxidizedat high temperature in a subsequent melting furnace 24 together withchar, whereby unburnt solids are decomposed and resultant ash content isconverted to molten slag 25. The hot combustion gas is fed into a wasteheat boiler 2, where it is cooled to 450-650° C. with heated water 19coming from economizer 6, thereby recovering saturated steam 20 having atemperature of about 300° C. and a pressure of about 80 kg f/CM².Subsequently, the combustion gas is supplied to a medium-temperaturefilter 3 for dust filtration in a temperature range of 450-650° C. at afiltration velocity of 1-5 cm/sec under a pressure of from −5 kPa (gage)to +2 kPa (gage). A neutralizing agent 13 such as slaked lime isintroduced into a flue 12 connecting to the medium-temperature filter 3so that the HCl in the combustion gas is removed by absorption. Stream14 which is part or all of the combustion gas exiting themedium-temperature filter 3 is supplied to a heating furnace 4, where itis reburnt with an auxiliary fuel 15 and thereby heated to a highertemperature. The thus heated combustion gas 16 is directed to a steamsuperheater 5, where the saturated steam 20 coming from the waste heatboiler 2 is superheated to about 500° C. The combustion gas 17 exitingthe steam superheater 5 goes to the economizer 6 and an air heater 7 forfurther heat recovery. Thereafter, the combustion gas passes through aninduced blower 8 and is discharged from a stack 9.

The steam 21 superheated in the steam superheater 5 is sent to a steamturbine 22 for generating electricity 28. The auxiliary fuel can besaved by the same method as described in connection with the systemshown in FIG. 1.

Denoted by 11 and 18 in FIG. 2 are noncombustibles and water.

FIG. 3 is a flow sheet for a heat recovery system employing thecombination of fluidized-bed gasification and slagging gasification withreburning by firing of a combustible gas in accordance with yet anotherembodiment of the invention.

A fluidized-bed gasification furnace (low temperature gasifier) 30employs a small air ratio and the temperature of the fluidized bed isheld as low as 450-650° C. such that the gasification reaction willproceed at a sufficiently slow rate to produce a homogeneous gas. In aconventional incinerator, the combustion temperature is so high thataluminum (m.p. 660° C.) will melt and be carried with the exhaust gas asfly ash, whereas iron and copper are oxidized so that they have only lowcommercial value when recycled. In contrast, the fluidized-bedgasification furnace 30 has a sufficiently low fluidized-bed temperatureand yet has a reducing atmosphere so that metals such as iron, copperand aluminum can be recovered in an unoxidized and unadulterated statewith the combustible material having been gasified, such that the ashmetals are suitable for material recycling.

A swirl melting furnace (high temperature gasifier) 31 has a verticalprimary combustion chamber, an inclined secondary combustion chamber anda slag separating section. A char-containing gas blown into the furnaceis burnt at high temperature while it swirls together with combustionair, whereas molten slag 25 on the inside surface of the furnace wallflows into the secondary combustion chamber and thence flows down theinclined bottom surface. In the slag separating section, a radiationplate maintains the slag temperature and thereby enables a consistentslag flowout 25.

Thus, the combustible gas and char that have been generated in thegasification furnace 30 are gasified at a high temperature of about1,350° C., and thresh content thereby is converted to molten slag whileensuring complete decomposition of dioxins and the like.

The hot gas from the melting furnace 31 enters a waste heat boiler 32;such hot gas contains unburnt gases such as hydrogen and methane and iscooled to 450-650° C. in boiler 32, whereby steam is recovered.Thereafter, the gas is passed through a medium-temperature filter 33 toremove dust such as solidified salts in a temperature range of 450-650°C. at a filtration velocity of 1-5 cm/sec under a pressure of from −5kPa (gage) to +2 kPa (gage). The dust-free gas then enters a heater 34which is supplied with air, oxygen or the like to reburn the gas withoutfeed of external fuel. It should be noted that the applicability of themethod shown in FIG. 3 is limited to wastes 10 having a high heat value.

Denoted by 35, 36 and 37 respectively are an economizer, an airpreheater and a steam turbine for high efficiency power generation.

FIG. 4 is a flow sheet for a combined cycle power plant employinggasification and slagging gasification in accordance with a furtherembodiment of the invention. As shown in FIG. 4, municipal wastes 10 aregasified in a gasifying furnace 23 to generate combustible gas which,together with char, is oxidized at high temperature in the subsequentmelting furnace 24, where the ash content is converted to molten slag25. The hot combustible gas is supplied to a waste heat boiler 2, whereit is cooled to 450-650° C. by heat exchange with heated water 19 comingfrom an economizer 6 so as to recover saturated steam having atemperature of about 300° C. and a pressure of about 80 kgf/CM². Thecombustible gas is then passed through a medium-temperature filter 3 fordust filtration in a temperature range of 450-650° C. at a filtrationvelocity of 1-5 cm/sec under a pressure of from 102 kPa (gage) to 5 MPa.A neutralizing agent 13 such as slaked lime is introduced into a flue 12connecting to the medium-temperature filter 3 such that the HCl in thegas is removed by absorption. All of the steps described up to here areperformed within a pressure vessel 26. Stream 14 of the combustible gasexiting the filter 3 is supplied, together with combustion air 15, intoa gas turbine 27 for generating electricity 28. Exhaust gas 16 from thegas turbine 27 is fed into a steam superheater 5, where the steam 20coming from the waste heat boiler 2 is superheated to 500° C. and isthence supplied to the economizer 6 and an air preheater 7 for heatrecovery. Thereafter, the exhaust gas is passed through an inducedblower 8 and discharged from a stack 9. The steam 21 exiting the steamsuperheater 5 is sent to a steam turbine 22 for generating electricity28.

Denoted by 18 in FIG. 4 is water.

FIG. 7 is a flow sheet for a test plant, with test data included, thatwas operated to evaluate the effectiveness of a medium-temperaturefilter in preventing the corrosion of heat transfer pipes whilesuppressing dioxin (DXN) resynthesis. When the medium-temperature filter13 was to be used, it was in the form of a honeycomb filter made of analumina-based ceramic material and the combustion gas was passed throughthis filter to remove dust at 500° C.

When no medium-temperature filter was used, the steam temperature was500° C. and the service life of the heat transfer pipes in the steamsuperheater 5 was 2,000 hours. By allowing the combustion gas having atemperature of 900° C. to pass through a radiation boiler 2, the DXNconcentration was reduced by about 35%. On the other hand, passingthrough the steam superheater+boiler (5+2), economizer 6 and airpreheater 7, resulted in DXN being resynthesized to have itsconcentration increased to at least 200 ng. TEQ/Nm³. Therefore, the DXNwas removed together with dust by passage through a bag filter 38 and ascrubber 39 before the combustion gas was discharged from a stack 9.

When the medium-temperature filter 3 was used, the steam temperature was500° C. and the service life of the heat transfer pipes in the steamsuperheater 5 was 4,000 hours, accompanied by a 0.1 mm reduction in pipethickness. There was no detectable DXN resynthesis.

If one attempts to increase the steam temperature with a view toimproving the efficiency of power generation by a steam turbine,corrosion by molten salts and the like in the combustion gas isaccelerated in a heat transfer pipe of a temperature in excess of about400° C. and, hence, the steam temperature must be heated below 400° C.

In contrast, by using the medium-temperature filter to remove the moltensalts in the combustion gas or combustible gas before it enters thesteam superheater, the corrosion of the heat transfer pipes issufficiently suppressed that the steam temperature can be raised toabout 500° C., thereby improving the efficiently of power generation.

In accordance with the present invention, the salts in a combustion gasor combustible gas are removed by performing dust filtration at atemperature of 450-650° C. which enables the solidification of moltensalts. Therefore, the dust-free combustion gas or combustible gas can besufficiently reburnt and heated without causing the corrosion of heattransfer pipes in a superheater. This contributes to an improvement inthe efficiently of power generation using combustion gases produced byburning municipal waste and/or RDF.

If this technology is combined with a dechlorination method usingneutralizing agents, the corrosive nature of such combustion gases orcombustible gases can be further reduced by a significant degree. Inaddition, the resynthesis of dioxins can be suppressed.

We claim:
 1. An apparatus for recovering heat and generating power fromwastes, said apparatus comprising: a low temperature gasifier forgasifying wastes at a low temperature to thereby produce low temperaturecombustible gas; a high temperature gasifier for gasifying the lowtemperature combustible gas at a high temperature to produce hightemperature combustible gas containing at least one of alkali metalchlorides, calcium chloride, copper oxide and copper chloride; a wasteheat boiler for cooling the high temperature combustible gas and forproducing steam; a dust filter for filtering the thus cooled combustiblegas at a temperature of from 450-650° C. to thereby remove therefrom thealkali metal chlorides, calcium chloride, copper oxide and copperchloride as solid materials; a furnace for reburning the thus filteredcombustible gas with air, oxygenated air or oxygen to thereby reheat thegas to a temperature sufficient to permit heat recovery therefrom; asteam superheater to receive the thus reheated gas and the steam and tosuperheat the steam by recovery of heat from the gas; and a steamturbine to receive the thus superheated steam and thereby to generatepower.
 2. An apparatus as claimed in claim 1, wherein said dust filtercomprises a ceramic filter.
 3. An apparatus as claimed in claim 2,wherein said filter supports a denitration catalyst.
 4. An apparatus asclaimed in claim 1, wherein said filter supports a denitration catalyst.5. An apparatus as claimed in claim 1, further comprising at least oneof an economizer and a preheater to recover heat from the gas dischargedfrom said steam superheater, and means for then discharging the gas tothe atmosphere.
 6. An apparatus as claimed in claim 1, furthercomprising means for introducing neutralizing agent into at least one ofthe wastes and the combustible gas prior to filtration by said dustfilter.